Many present hydrostatic transmissions include a variable displacement hydraulic pump driven by an input shaft. A hydraulic fluid is pumped from the hydraulic pump to a hydraulic motor for driving an output shaft. In these devices, there is no mechanical linkage between the input shaft which drives the hydraulic pump and the output shaft which is driven by the hydraulic motor. The hydraulic pumps and motors used in hydrostatic transmissions are typically axial piston devices which use a small amount of fluid for internal lubrication, which in turn results in fluid being lost from the hydrostatic circuit. In order to replenish fluid lost from the hydrostatic loop during operation of the transmission, a fixed displacement charge pump is driven by the variable displacement pump to communicate a supply of fluid in a charge flow path from a fluid reservoir to the hydrostatic circuit.
A desirable feature in the transmission of a working vehicle is the capability of transmitting fluid power for driving auxiliary implements, such as lift cylinders, steering valves, and the like. In some instances, this is accomplished by means of a dedicated hydraulic pump external to the hydrostatic transmission for supplying a pressurized auxiliary flow to an auxiliary implement fluid circuit. This approach is costly and mechanically redundant. More commonly, the fixed displacement charge pump which replenishes fluid in the hydrostatic circuit is used also to supply the auxiliary flow. This type of arrangement eliminates the expense, manufacturing, and maintenance detriments associated with the provision of an additional pump.
While the above approach minimizes the mechanical complexity of the system, additional problems exist which detrimentally impact the overall efficiency of the transmission. Today, there exists two commonly employed arrangements for a hydrostatic transmission in which a single pump is utilized to provide system charge pressure as well as providing auxiliary implement flow. Each of these arrangements have significant drawbacks which affect hydrostatic transmission performance and hydrostatic transmission life.
In one arrangement, the charge flow path supplies oil for auxiliary functions, i.e., to the auxiliary implement circuit, before it is available to the hydrostatic closed loop. At high auxiliary pressure requirements the charge pump leaks substantially, and when the leakage is great enough that the charge pump cannot maintain enough flow to replace the leakage in the hydrostatic closed loop, the hydrostatic transmission component life is severely affected and may cause premature failure.
To circumvent this problem, and as proposed in the second currently used arrangement, the charge flow path supplies fluid first to the hydrostatic closed loop and then the auxiliary circuit. This creates, however, two additional problems. At high auxiliary function pressure requirements the charge pressure on the side of the hydrostatic loop which communicates with the auxiliary circuit, traditionally the "low" side of the loop, is a sum of the auxiliary pressure and the pressure setting of a charge relief valve. The addition of auxiliary pressure to the low side of the hydrostatic loop increases loading on shaft bearings and rotated components, thereby decreasing transmission life.
An additional problem lies in the fact that added auxiliary pressure on the low side of the hydrostatic loop tends to act as a brake. The braking action relates to an increased torque necessary to generate working pressure in the pump and the motor, and, in turn, lowers the torque efficiency of the pump and motor thereby increasing horsepower loss and heat generation.
The present invention is directed toward overcoming one or more of the problems set forth above.